Green’s Dictionary of Slang

(Aus.) something the name of which one does not know or has forgotten.

[1825		‘Thinks I to Myself, Thinks I’ in Universal Songster I 25/2: My dear Mrs Whifmejig, I am so glad to see you].
1999		G. Seal Lingo 72: There are even Lingo names for those things we cannot remember the names of – whatsit, dooverlackey (or just doover), whatchamacallit, thingumebob, thingy, whosamajig, whatasamajig, thingumajig, whatsit, what’s-his-face and whosemawhat.